Engines used to power lawn, garden, and forestry equipment, such as chain saws, are often used in environments in which there is a large concentration of sawdust, dirt, and particulate contaminants. Without an air filter, dirt, dust, and other solid airborne contaminants drawn into the carburetor can become embedded in the oil film between the moving engine parts, thus creating an abrasive media that promotes engine failure. In environments having high concentrations of airborne contaminants, air filters clog more rapidly, thus obstructing air flow and decreasing engine performance unless changed frequently. Where dust and dirt are severe, larger filters are used to extend intervals between filter replacement. However, size and space on portable, hand-held equipment is a premium, and thus larger filters are somewhat undesirable. To enable use of smaller filters, without reducing maintenance intervals to replace filters, the carburetor and/or air filter of the engine are enclosed in a box that is pressurized with clean air to keep dirt and other contaminants away from the carburetor and its air filter. If the engine is fuel injected and has no carburetor, the pressurized box with an air filter is coupled to the intake port of the cylinder. The slight pressurization of the intake air also helps to improve performance of the internal combustion engine by delivering pressurized air to the carburetor so that the gas mixture load can be increased.
Two-stroke engines used on portable hand-held power equipment are air-cooled using air blown by a flywheel fan connected to the engine's crankshaft. Because weight, size and cost are critical constraints on design of engines for hand-held power tools, the relatively high velocity cooling air blown by the fan is a desirable source for air to pressure the carburetor or carburetor box. Unfortunately, air drawn into the fan is itself often contaminated with a considerable amount of dust or debris.
Two patents, U.S. Pat. No. 4,851,920 to Andreasson, et al. and U.S. Pat. No. 4,716,860 to Henriksson, et at., are examples of one approach to diverting a relatively contaminant-free portion of a high velocity air stream from fan to a carburetor. A plate along a portion of the periphery of a rotating fan impeller and parallel to the axis of rotation shields an inlet, located immediately behind the plate, to a duct leading to the carburetor port. The impeller imparts to particulate contaminants a centrifugal force that carries the contaminants radially away from the impeller. Due to the relatively higher inertia of the particles as compared to the air, the centrifugal forces carry the particles radially outwardly. Heavier particles, due to their inertia, do not easily bend around the plate and into the port. A stream of relatively high velocity air, relatively free of larger particles, is, however, capable of turning into the port and is thus diverted into the inlet. The port is connected by a tube to the carburetor. The particulate contaminants are carried away by the remainder of the air stream. A Jonsered Model 2051 chain saw improves on the Andreasson et at. design by placing a small ramp or "bump" at the leading edge of the plate, near the rotating fan blades. The bump on the leading edge of the duct inlet produces a lift which assists in deflecting airborne particulate contaminants away from the inlet.
There are problems with this design. The carburetor inlet duct in the flywheel fan housing interferes with the flow of cooling air out of the fan housing or volute. This interference reduces cooling of the engines. Under heavy loads and in the hot environment in which forestry, lawn and garden equipment sometime operate, the interference leads to engine overheating and failure. Due to its position, mounting of this design tends to be less than stable. Furthermore, the design is complicated and expensive to manufacture and assemble. It requires several additional components that must be manufactured to close tolerances and precisely aligned during assembly for satisfactory performance.
U.S. Pat. No. 4,233,043 to Catterson also relies on the relatively high inertia of the particulate matter to supply clean air to a carburetor duct. A duct projects into the side of a flywheel fan housing. The inlet to the duct is angled with respect to the flow of air so that the air being blown past the duct must undergo an abrupt change in direction in order to enter the duct. Particulate contaminants entrained in the air stream tend to be carried past the duct inlet due to their relatively high velocity. However, the Catterson et al. design is not able to provide a relatively high-velocity air stream for pressurizing a carburetor.